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<h1 id="patching">Patching</h1>

<h2 id="auxiliary-patch-methods">Auxiliary patch methods</h2>
<p>If you use manual patching, you are in full control of your state and the execution and handling of extra resources and logic. For annotation patching, Harmony offers you a number of methods you can implement on a patch class that allow you to execute code before and after patching on that class as well as methods that combine annotations with manually defining which methods should be patched.</p>
<p>Each of those methods can take up to three optional arguments that are injected by type so you can call them anything you like:</p>
<ul>
<li><code>MethodBase original</code> - the current original being patched</li>
<li><code>Harmony harmony</code> - the current Harmony instance</li>
<li><code>Exception ex</code> - only valid in <code>Cleanup</code> and receives a possible exception</li>
</ul>
<p>Here is a simple example that patches a method inside a private type:</p>
<pre><code class="lang-csharp" name="example">[HarmonyPatch] // at least one Harmony annotation makes Harmony not skip this patch class when calling PatchAll()
class MyPatch
{
    // here, inside the patch class, you can place the auxiliary patch methods
    // for example TargetMethod:

    public static MethodBase TargetMethod()
    {
        // use normal reflection or helper methods in &lt;AccessTools&gt; to find the method/constructor
        // you want to patch and return its MethodInfo/ConstructorInfo
        //
        var type = AccessTools.FirstInner(typeof(TheClass), t =&gt; t.Name.Contains(&quot;Stuff&quot;));
        return AccessTools.FirstMethod(type, method =&gt; method.Name.Contains(&quot;SomeMethod&quot;));
    }

    // your patches
    public static void Prefix()
    {
        // ...
    }
}
</code></pre><h3 id="prepare">Prepare</h3>
<p>Before the patching, Harmony gives you a chance to prepare your state. For this, Harmony searches for a method called</p>
<pre><code class="lang-csharp">static void Prepare(...)
static void Prepare(MethodBase original, ...)
static bool Prepare(MethodBase original, ...)
// or
[HarmonyPrepare]
static void MyInitializer(...)
static void MyInitializer(MethodBase original, ...)
static bool MyInitializer(MethodBase original, ...)
</code></pre>
<p>By returning <code>false</code> it can skip the patching in this class. It is recommended to inject <code>original</code> into Prepare because <strong>Harmony calls it at least twice</strong>: once before patching in the class starts (original is <code>null</code>) and once for each method being patched (original indicates which method is currently patched).</p>
<h3 id="targetmethod">TargetMethod</h3>
<p>Most of the times, you will use a combination of <code>HarmonyPatch()</code> annotations on the class to define the method you want to patch. Sometimes though, it is necessary to calculate the method with code. For this, Harmony searches for a method called</p>
<pre><code class="lang-csharp">static MethodBase TargetMethod(...)
// or
[HarmonyTargetMethod]
static MethodBase CalculateMethod(...)
</code></pre>
<p>That method, if it exists, is expected to return a <code>MethodBase</code> of the method to be patched. The return cannot be null. If you want to conditionally skip patching, use a <code>Prepare()</code> method.</p>
<h3 id="targetmethods">TargetMethods</h3>
<p>If you want to patch multiple methods with the same patch, you can use <code>TargetMethods</code>. It has the same behaviour as <code>TargetMethod</code> except that it returns an enumeration of <code>MethodBase</code> instead of a single <code>MethodBase</code>:</p>
<pre><code class="lang-csharp">static IEnumerable&lt;MethodBase&gt; TargetMethods(...)
// or
[HarmonyTargetMethods]
static IEnumerable&lt;MethodBase&gt; CalculateMethods(...)
</code></pre>
<p>A typical implementation would <code>yield</code> the results like this:</p>
<pre><code class="lang-csharp" name="example">static IEnumerable&lt;MethodBase&gt; TargetMethods()
{
    // if possible use nameof() or SymbolExtensions.GetMethodInfo() here
    yield return AccessTools.Method(typeof(Foo), &quot;Method1&quot;);
    yield return AccessTools.Method(typeof(Bar), &quot;Method2&quot;);

    // you could also iterate using reflections over many methods
}
</code></pre>
<p>Similar to <code>TargetMethod</code>, the implementation cannot return zero results. If you want to conditionally skip patching, use a <code>Prepare()</code> method.</p>
<h3 id="cleanup">Cleanup</h3>
<p>After patching, Harmony gives you a chance to clean up your state. For this, Harmony searches for a method called</p>
<pre><code class="lang-csharp">static void Cleanup(...)
static void Cleanup(MethodBase original, ...)
static Exception Cleanup(MethodBase original, ...)
// or
[HarmonyCleanup]
static void MyCleanup(...)
static void MyCleanup(MethodBase original, ...)
static Exception MyCleanup(MethodBase original, ...)
</code></pre>
<p>Similar to <code>Prepare()</code> this method is called with <code>original</code> set to the method that just has been patched and then finally one more time before ending the overall patching (original will be <code>null</code>).</p>
<p>Additionally, you can intercept exceptions that are thrown while patching. Use the injection of <code>exception</code> to learn what happened and check if you can cast it to <code>HarmonyException</code> to get more information. Finally, you can return <code>Exception</code> to replace the exception or <code>null</code> to suppress it.</p>
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